Summary Medical device infections (MDI) caused by vancomycin resistant Enterococcus (VRE) are associated with a high rate of treatment failure and increased mortality. Infections due to vancomycin-resistant Enterococcus faecium (VREF) are more problematic than any other species of enterococci since these organisms are associated with the highest rate of vancomycin resistance and are often multi-drug resistant making treatment more difficult due to the limited available antimicrobial options. MDI are one of the most difficult infections to treat because of the high association with biofilm producing pathogens, which represents a significant barrier for effective antibiotic therapy. Daptomycin, a novel lipopeptide antibiotic, rapidly penetrates biofilms and exerts bactericidal activity against metabolically active or arrested enterococci, including VREF. The daptomycin dose for VRE to optimize patient outcomes and prevent the emergence of resistance during MDI, however, is currently unknown. In addition, there is little to no information regarding the optimal daptomycin drug combination to treat VRE MDI. Therefore, there are two potential strategies to optimize daptomycin therapy for VRE MDI. One is daptomycin dose optimization and the other strategy is the use of combination therapy. The long-term goal is to optimize patient outcomes and preserve daptomycin therapy for VRE MDI infections through utilization of the ideal dose exposure to prevent daptomycin resistance in enterococci. The overall objective for this study is to define the dose-exposure breakpoint (pharmacokinetic/pharmacodynamic [PK/PD] breakpoint) for daptomycin resistance prevention in biofilm embedded VREF and the correlating breakpoint when daptomycin is combined with other antimicrobials. The central hypothesis is that higher daptomycin dose exposures alone or in antibiotic combination are needed against biofilm embedded VREF to prevent the emergence of resistance compared to dose exposures using planktonic VREF. The rationale behind the proposed research is that data on the daptomycin dose relationship with biofilm embedded enterococci will lead to clinical dose optimization, improved patient outcomes, reduced emergence of resistance, and preservation of daptomycin as a viable antibiotic for clinical use. The central hypothesis will be tested by pursuing two Specific Aims: 1) Determine the dose-exposure breakpoints for daptomycin resistance using biofilm embedded molecularly defined and clinical strains of VREF to determine the optimal dose; and 2) Identify the optimal dose-exposure of daptomycin in combination with ampicillin or rifampin that is associated with the prevention of the development of VREF resistance. The proposed research is innovative because we will utilize an in vitro biofilm PK/PD model that simulates drug exposures in humans. This technique allows for frequent assessment of antibiotic activity as well as observation of changes in the organism susceptibility as it relates to specific drug exposures over time. The research proposed in this application is significant because it is expected to provide the knowledge needed to understand the resistance characteristics of biofilm embedded enterococci and their relationship to daptomycin dose exposure that will lead to dose optimization resulting in improved patient outcomes, and preservation of daptomycin as a viable therapeutic option for the treatment of enterococcal MDI.